pt., 19 lis 2021 o 14:48 Nicolas Weidmann via Std-Proposals
<std-proposals_at_[hidden]> napisał(a):
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> Hello,
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> Currently most templates can be fully specialized, however, alias templates cannot.
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> As the following examples illustrate, the problem alias templates specialization would solve can currently be mimicked using alias traits.
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>
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> But adding alias templates specialization would render to code more straightforward and readable:
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> The first example uses a default alias template followed by specializations.
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> The second example ‘deletes’ the default alias template, only allowing for specialized versions.
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> The first example achieved using traits:
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> ///////////////////////////////////////////////////////////////
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> template<typename T> struct AliasTrait {
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> using type = std::set<T>;
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> };
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> template<> struct AliasTrait<int> {
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> using type = std::vector<int>;
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> };
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> template<> struct AliasTrait<double> {
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> using type = std::list<double>;
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> };
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> template<typename T> void foo()
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> {
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> typename AliasTrait<T>::type values;
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> }
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> The first example achieved using alias template specialization:
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> ///////////////////////////////////////////////////////////////
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> template<typename> using alias = std::set<T>;
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> template<> using alias<int> = std::vector<int>;
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> template<> using alias<double> = std::list<double>;
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> template<typename T> void foo()
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> {
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> alias<T> values;
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> }
>

Why not simply:
```
template<typename> using alias = typename AliasTrait<T>::type;
```
And this was one of the reasons why we have aliases.
What will we grain exactly if we make aliases more complex?

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> The second example achieved using traits:
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> ///////////////////////////////////////////////////////////////
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> template<typename> structNoDefaultAliasTrait;
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> template<> struct NoDefaultAliasTrait<int> {
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> using type = std::vector<int>;
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> };
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> template<> structNoDefaultAliasTrait<double> {
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> using type = std::list<double>;
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> };
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> template<typename T> void bar()
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> {
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> typename NoDefaultAliasTrait<T>::typevalues;
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> }
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> The second example achieved using alias template specialization and introducing ‘deletion’ of the generic alias template:
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> ///////////////////////////////////////////////////////////////
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> template<typename> using no_default_alias= delete;
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> template<> using no_default_alias<int> = std::vector<int>;
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> template<> using no_default_alias<double> = std::list<double>;
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> template<typename T> void bar()
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> {
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> no_default_alias<T> values;
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> }
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> Regards,
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>
> Nicolas
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>
> 
> 
>
> Sent from my iPhone
> --
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